A variometer is an instrument which measures the speed of vertical ascent or descent. Variometers are used by pilots of aircraft and gliders in addition to enthusiasts of paragliding and hang gliding. These instruments are viewed as a critical part of the instrument display panel in aircraft, because they provide pilots with some very important information. For gliders, a variometer is useful for safety and for the purpose of steering, allowing people to track thermals more effectively.

Since humans do not fly under their own power, they do not have biological mechanisms which are designed to help them detect ascents or descents. While a rapid ascent or descent is usually noticeable, small variations are not, and pilots historically got into trouble because they had difficulty maintaining level flight. While it is possible to determine where one is in the sky with the use of a physical landmark, this is rather limiting. Variometers allow pilots to tell which direction they are moving in without the need to refer to a landmark in the area.

The way in which these devices work is quite simple. As aircraft rise or fall, the air pressure changes. A variometer is sensitive to pressure, providing a continuous and instant readout of the rate of ascent or descent which can be used during flight. These devices are not used as much during takeoff and landing, since pilots are presumably aware that they are rising or descending during these activities.

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Also known as a vertical speed indicator, a variometer has a readout which indicates whether someone is rising or falling, and at what speed. The speed may be indicated in meters, feet, or knots, depending on the instrument maker and the pilot's preferences. Pilots use the variometer to monitor their position in the sky. The device can also serve as an alert if a pilot enters a pocket of air, as the pilot may rise or fall rapidly.

Some variometers provide audio feedback to pilots. The device may be programmed to sound an audible tone to draw the pilot's attention to a notable reading, or it may continuously produce sound to let pilots known when they are rising and falling, and at what speed. Variometers can also be bundled into other types of aviation instruments, such as altimeters which indicate height. The device can also feature an averager which averages readings in an area, allowing a pilot to get a more generalized view of what is going on in the sky where he or she is.

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